2 years visit visa

[c4551m]

Hi, my question is about the standard visit visa for 2 years for my parents applying from Abu Dhabi.
My dad been here few time ( 2007, 2008, 2013,2016) now he's planning to visit us once a year. I am here since 2004 with my family.

Is He ok to apply for 2 years visa ? Instead of applying 6 months visa every time.

Will the documents be same as for 6 months visit visa ?

Could someone advice me on 2yrs or 5 years visa process.

Thanks.